System and method for determining tire force

ABSTRACT

A system and method for determining the magnitude of a force acting on a tire are disclosed. Further, a sensor and a tire suitable for use in the system and method are also disclosed. The system comprises: a tire strain sensor mounted on the tire, for detecting a tire strain at the mounted position, and generating data representing tire strain; a sensor locator for locating the sensor on the tire; a memory in which data on the relationship between the tire strain and force acting on the tire at each of measuring points are stored; and a processor computing the magnitude of the force, using the data representing tire strain obtained from the tire strain sensor and the data on the relationship acquired from the memory based on the data on the sensor location obtained from the sensor locator. The method comprises: obtaining data on tire strain from a tire strain sensor mounted on the tire; locating the tire strain sensor, to obtain data on the position of the tire strain sensor; obtaining data on a relationship between the tire strain and the force at the located position of the tire strain sensor, from previously stored data on relationships between the tire strain and force at a plurality of sensor positions; and computing the force using the obtained data on the tire strain and the obtained data on the relationship between the tire strain and the force.

This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 2003-366299 and 2004-217088 filed inJapan on Oct. 27, 2003 and Jul. 26, 2004 respectively, the entirecontents of which are hereby incorporated by reference.

The present invention relates to a system and method for determiningtire force acting on a vehicle tire.

In recent years, the number of vehicles equipped with computer-aidedvehicle control systems (CAVCS) such as anti-lock brake system, tractioncontrol system, vehicle stability control system, attitude controlsystem, suspension control system and steer-by-wire system is rising.

In such control systems (CAVCS), if data on the magnitude and directionof the force acting on a rolling tire can be utilized, control accuracywill be dramatically improved. Hitherto, however, there is no way to getdata on the forces acting on rolling tires during running. In the carindustry, therefore, there is a great demand for a device which canmonitor the forces acting on vehicle tire during running.

A primary object of the present invention is therefore, to provide asystem and method by which a force acting on a vehicle tire duringrunning can be easily determined.

Another object of the present invention is to provide a strain sensordurable against large deformation and suitable for use on a tire.

Still another object of the present invention is to provide a pneumatictire provided with a strain sensor durable against large tiredeformation.

According to one aspect of the present invention, a system fordetermining the magnitude of a force acting on a tire, comprises:

a tire strain sensor mounted on the tire, for detecting a tire strain atthe mounted position, and generating data representing tire strain;

a sensor locator for locating the sensor on the tire;

a memory in which data on the relationship between the tire strain andforce acting on the tire at each of measuring points are stored; and

a processor computing the magnitude of the force, using the datarepresenting tire strain obtained from the tire strain sensor and thedata on the relationship acquired from the memory based on the data onthe sensor location obtained from the sensor locator.

According to another aspect of the present invention, a method ofdetermining the magnitude of a force acting on a tire comprises:

obtaining data on tire strain from a tire strain sensor mounted on thetire; locating the tire strain sensor, to obtain data on the position ofthe tire strain sensor;

obtaining data on a relationship between the tire strain and the forceat the located position of the tire strain sensor, from previouslystored data on relationships between the tire strain and force at aplurality of sensor positions; and

computing the force using the obtained data on the tire strain and theobtained data on the relationship between the tire strain and the force.

Embodiments of the present invention will now be described in detail inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of a tire for explaining variousforces acting on a tire and the coordinate system used to express themeasuring position;

FIG. 2 and FIG. 3 are a schematic cross sectional view of a pneumatictire and a side view of the tire, respectively, for explainingrelationships between tire forces and measuring positions;

FIGS. 4( a), 4(b) and 4(c) are graphs each showing a radial strain ∈sand circumferential strain ∈t as a function of a force Fx, Fy or Fz;

FIG. 5( a) is a graph showing a shear strain as a function of ameasuring position (degree around the tire rotational axis);

FIG. 5( b) is a graph showing a shear strain at zero-degree position asa function of a force Fx;

FIG. 6 is a schematic perspective view of a tire strain sensor accordingto the present invention;

FIGS. 7, 8 and 9 each show a magnet and sensor element arrangement forthe tire strain sensor;

FIG. 10 is a cross sectional view of a pneumatic tire showing a sensorposition and also showing a typical structure of a passenger car radialtire;

FIGS. 11 and 12 are cross sectional views each showing another sensorposition;

FIG. 13 is a diagram showing a computer-aided vehicle control systems(CAVCS) according to the present invention;

FIG. 14 is a diagram for explaining the measuring points;

FIG. 15 is a diagram for explaining a sensor arrangement;

FIGS. 16, 17 and 18 are diagrams each showing a tire force determiningsystem (TFDS) according to the present invention;

FIG. 19 shows the arrangement of the sensors and the arrangement of themeasuring points and induction coil COL used in the example shown inFIG. 18; and

FIGS. 20 and 21 are diagrams each showing a further example of the tireforce determining system (TFDS) according to the present invention.

BASIC PRINCIPLES

FIG. 1 shows various forces acting on a rolling tire: a force in theback-and-forth-direction x (hereinafter back-and-forth-direction forceFx); a force in the lateral direction y (hereinafter lateral force Fy);a force in the vertical direction z (hereinafter vertical force Fz); anda force Fyz, Fzx, Fxy around an axis x, y, z passing the center of thetire in each of the above-mentioned three directions.

As far as there is a linear correlation or almost linear correlationbetween an applied force and the resultant tire strain or stress(hereinafter, simply referred as tire strain), there is a possibility ofdetermining the various forces by the use of measured tire strains.

Taking forces Fx, Fy and Fz in three translational motion directions asan example, the tire strain will be explained.

As shown in FIG. 2 and FIG. 3, in the circumferential center of theground contacting patch of a tire, with respect to four measuringpositions Pa, Pb, Pc and Pd of a pneumatic tire 1 (Pa at the tireequator, Pb at the tread edge, Pc in the sidewall portion and Pd in thebead portion), the strain ∈s in the radial direction and the strain ∈tin the circumferential direction were measured at the tire outersurface, applying to the tire a back-and-forth-direction force Fx orlateral force Fy or vertical force Fz by turns.

The results are shown in the following Table 1.

TABLE 1 Measuring position Pa Pb Pc Pd Correlation with Fx εs D C D D εtC C B D Correlation with Fy εs C D D B εt D C A D Correlation with Fz εsC D B B εt C D B D A: linear, large variation B: linear, small variationC: nonlinear, small variation D: absent

At the positions Pa and Pb in the tread portion, each strain ∈s, ∈t hassubstantially no correlation or a small nonlinear correlation with theforces Fx, Fy and Fz. Thus, it is very difficult or almost impossible todetermine the forces Fx, Fy and Fz from the strains at the positions Paand Pb.

At the position Pd in the bead portion, the strain ∈s in the radialdirection has a linear correlation with each of the lateral force Fy andvertical force Fz, but the strains ∈s and ∈t each have no correlationwith the back-and-forth-direction force Fx. Therefore, it is almostimpossible to know all the forces Fx, Fy and Fz from the strains at theposition Pd.

On the other hand, at the position Pc in the sidewall portion, at leastone of the strains ∈s and ∈t has a linear correlation with each of thethree translational forces Fx, Fy and Fz.

Therefore, there is a possibility that the strains measured at theposition Pc can be used to determine the three translational forces Fx,Fy and Fz in order to know the state of a rolling tire.

FIGS. 4( a), 4(b) and 4(c) show the radial and circumferential strains∈s and ∈t at the position Pc as a function of the forces Fx, Fy and Fz,respectively. As shown in FIG. 4( a), although theback-and-forth-direction force Fx and strain ∈t show a linearcorrelation, the variation is so small to make accurate determination ofthe force.

Therefore, the present inventors explored a possibility of utilizinganother parameter instead of the radial and circumferential strains, andconfirmed that a shear strain or shearing stress (hereinafter simplyshear strain ∈γ or strain ∈γ) is suitable for determining the tireforces.

For example, FIG. 5( a) shows the variation of the shear strain ∈γmeasured at a fixed point on the outer surface of a tire at theabove-mentioned position Pc during rotating the tire while applying aconstant force Fx. The measurement was made changing Fx from 0 to 3.5kN. The strain ∈γ has a linear or almost linear correlation with Fx forexample as shown in FIG. 5( b) which shows the shear strain ∈γ at 0degree (6 o'clock position) as a function of Fx.

Further, it was also confirmed that the shear strain ∈γ has a relativelylarge and linear or almost linear correlation with each of the forces Fyand Fz as well although the detailed discussion is omitted here.

AS to the tire strain as being the measuring parameter to determine theforces Fx, Fy and Fz, the shear strain ∈γ at a position Pc in thesidewall portion is used in this embodiment to improve the accuracy.

<Strain Sensor>

AS the tire deformation repeated during running becomes very large inthe sidewall portion 3, a conventional wire resistance strain gauge isvery liable to be broken. Thus, it is problematic in view of durability.Therefore, the strain sensor S has to be durable against the repeatedlarge deformation and preferably has an ability to follow thedeformation.

In this invention, therefore, a new type of strain sensor S was thoughtup.

AS shown in FIGS. 6, 7, 8 and 9, the strain sensor S is made up of atleast one magnet 11, at least one magnetometric sensor element 12disposed in the magnetic flux of the magnet 11 and a spacer 13therebetween.

For the magnetometric sensor element 12, semiconductor magnetometricsensor elements such as hall element and magnetoresistance element (MR)can be used. Preferably, a hall element is used for the sensitivity andstability.

For precision and also convenience in handling, the magnet(s) 11 andsensor element(s) 12 are embedded in a resilient material as a solidelectronic part while leaving a space therebetween. In this case,therefore, the resilient material filling the space functions as theabove-mentioned spacer 13.

It is important for the strain sensor S to make an elastic deformationfollowing the deformation of the sidewall portion 3. Therefore, as theresilient material 13, various rubbery materials which is preferably notharder than the sidewall rubber to which the strain sensor S is attachedcan be used. For example, thermoplastic elastomers (TPE) suitable forcasting and injection molding are preferably used.

When the strain sensor S is formed as a solid electronic part by moldingor the like, it is preferable that the surface thereof is roughened ortreated to improve bonding to the tire rubber. As a furthermodification, in case of the sensor to be embedded in the tire sidewallportion during tire vulcanization, the sidewall rubber itself may beused as the above-mentioned resilient material into which the magnet(s)11 and sensor element(s) 12 are embedded.

When the strain sensor S is deformed under stress, the relative locationbetween the magnet(s) 11 and magnetometric sensor element 12 is changedand accordingly the flux at the magnetometric sensor elements 12 is alsochanged. Thus, the magnetometric sensor element 12 can generate ananalog output corresponding to the variation of the flux or the strain.

Three examples of the magnet and sensor element arrangement are shown inFIGS. 7 to 9.

In FIG. 7, the sensor S is made up of a single magnetometric sensorelement 12 and a single magnet 11. The sensor element 12 and magnet 11are arranged in line such that the element 12 faces towards a N (or S)pole of the magnet 11. In this example, the direction of the center lineon which the element 12 and magnet 11 are aligned becomes the mostsensitive direction N.

If the sensor element 12 has no directivity with respect to a directionaround the above-mentioned sensitive direction N, and further themagnetic flux density is substantially constant in the direction aroundthe direction N, then the sensor S as whole has no directivity withrespect to the direction around the most sensitive direction N.

However, if the sensitivity of the sensor element 12 or the magneticflux density has a directivity, there is a possibility that the sensorhas a directivity with respect to the direction around direction N. Insuch as a case, the directivity is oriented to the tire circumferentialdirection.

In general, the density of the magnetic flux is almost constant near thecenter line of the magnet 11. Accordingly, if the sensor element 12 ismade a small parallel displacement or tilt relatively to the magnet 11,as the variation of the magnetic flux density at the sensor element issmall, there is a tendency for the sensitivity to become low. To improvethe sensitivity, the sensor element 12 can be disposed off the centerline of the magnet 11 to avoid the almost constant flux zone.

FIG. 8 shows such example, wherein the sensor S is made up of a singlemagnet 11 and a plurality of (e.g. two) magnetometric sensor elements 12disposed off the center line (N) of the magnet 11. In this example, twosensor elements 12 and one magnet 11 are disposed on a plane such thatthe two sensor elements 12 are disposed one on each side of the centerline (N) symmetrically about the center line (N), and the sensorelements 12 are inclined to orient towards one pole N (or S). In thisexample, the variation of flux due to strain at one of the elements 12occurs inversely to that at the other element 12.

Therefore, the difference between the analog outputs of the two elements12 is used as the output of the sensor S. Accordingly, the output levelbecomes twice of one element and further increased by the off-centerarrangement, namely the sensitivity is effectively increased withrespect to the strain in the above-mentioned plane. Further, as a resultof the increase in the sensitivity in the above-mentioned plane, thesensitivity in a perpendicular plane relatively decreases. Thus, thesensor S as whole has such a directivity in the direction around themost sensitive direction N.

Furthermore, FIG. 9 shows another example wherein, a singlemagnetometric sensor element 12 and a plurality of (e.g. two) magnets 11are arranged on a plane. The sensor element 12 is disposed on the axisof symmetry about which two magnets 11 are disposed symmetrically. Withrespect to the positions of the NS poles, the two magnets 11 arereversed. The sensor element 12 is oriented perpendicularly to the axisof symmetry. In this example too, the sensitivity in the above-mentionedplane is increased, and as a result, the sensitivity in a perpendicularplane relatively decreases. Thus, the sensor S as whole has such adirectivity.

<Sensor Orientation>

As explained above, the strain sensor S has the most sensitive directionN.

In order to sense the shear strain component (∈γ) of the tire strain ∈in the sidewall portion 3, the strain sensor S is oriented such that themost sensitive direction N inclines at an angle (beta) in a range offrom 10 to 80 degrees, preferably 20 to 70 degrees, more preferably 30to 60 degrees, still more preferably 40 to 50 degrees with respect tothe tire radial direction.

Further, when the directivity of the sensor S around the axis in themost sensitive direction N shows a specific direction, the sensor S hasto be oriented so that the sensitivity to the shear in thecircumferential direction becomes maximum.

<Sensor Zone>

The above-mentioned position Pc has to be within a circumferentiallyextending annular zone Y which extends radially outwardly and inwardlyfrom the center M of the tire sectional height H by a radial distance Lof at most 25%, preferably less than 20%, more preferably less than 15%of the tire sectional height H as shown in FIG. 10.

In this zone Y, the measuring points MP for which strain sensors S aredisposed are provided (thus, hereinafter, sensor zone Y).

<Measuring Points>

The number (m) of the measuring points MP has to equal to or more thanthe number (n) of forces F to seek.

In this embodiment, the forces are Fx, Fy and Fz in three translationalmotion directions x, y and z. Therefore, three or more measuring pointsMP are provided at different circumferential positions around therotational axis of the tire.

As to the circumferential positions of the measuring points MP, in viewof the data processing following the measurements, it is preferable thatthe measuring point MP are arranged symmetrically about a straight line(for example z-axis or vertical axis) passing through the tirerotational axis or equiangularly around the tire rotational axis.

AS to the radial positions of the measuring points MP, usually they aredisposed on the circumference of a circle having the center on the tirerotational axis. In other words, they are disposed at the same radialdistance from the tire rotational axis. However, this does not mean toexclude that the measuring points MP are disposed at different radialpositions. For example, such a combination of the regularly arrangedmeasuring points MP for the forces Fx, Fy ad Fz and an irregularlyarranged sensor for another force or parameter is possible.

The sensors S are for example, disposed at a radial position between thecenter M and 15% radially inwardly therefrom.

<Pneumatic Tire>

Incidentally, the tire 1, a pneumatic tire comprises a tread portion 2,a pair of axially spaced bead portions 4 each with a bead core 5therein, a pair of sidewall portions 3 extending between the tread edgesand the bead portions, and a carcass 6 extending between the beadportions 4, a belt 7 disposed radially outside the carcass 6 in thetread portion 2.

In this embodiment, the tire 1 is a radial ply tire for passenger cars.

The carcass 6 comprises at least one ply 6A of cords arranged radiallyat an angle of 90 to 70 degrees with respect to the tire equator, andextending between the bead portions 4 through the tread portion 2 andsidewall portions 3, and turned up around the bead core 5 in each beadportion 4 from the inside to the outside of the tire so as to form apair of turned up portions 6 b and one main portion 6 a therebetween.

Between the main portion 6 a and turned up portion 6 b of the carcassply 6A in each bead portion, there is disposed a bead apex made of hardrubber extending radially outwards from the radially outside of the beadcore, while tapering towards its radially outer end to reinforce thebead portion.

The belt comprises a breaker 7 and optionally a band 9 on the radiallyoutside of the breaker.

The breaker 7 is disposed on the crown portion of the carcass andextends almost allover the tread width, and comprises at least two crossplies, a radially inner ply 7A and a radially outer ply 7B, each made ofcords laid parallel with each other at an angle of from 10 to 35 degreeswith respect to the tire equator so that the cords in one ply cross thecords in the other ply.

The band 9 is disposed on the radially outside of the breaker 7 and madeof at least one cord wound at almost zero angle or a small angle of lessthan 5 degrees with respect to the circumferential direction of thetire.

<Sensor Mounting Position>

On the other hand, in view of the sensibility to the tire strain, it ispreferable that the strain sensors S are disposed on or in the outersurface of the sidewall portion 3 as shown in FIG. 10. Further, as shownin FIG. 11, the strain sensors S can be disposed within the sidewallportion 3, for example, on the axially outside of the reinforcing cordlayer inclusive of the carcass 6. Furthermore, it may be possible todispose the sensor S on the inner surface of the sidewall portion 3 asshown in FIG. 12. In any case, it is preferable to the use of adhesiveagent that the strain sensors S are placed on the outer or inner surfaceor within the sidewall portion 3 before tire vulcanization and arebonded chemically and/or mechanically during vulcanization.

<Tire Force Determining System>

Next a tire force determining system TFDS according to the presentinvention is explained.

FIG. 13 shows a diagram of a computer-aided vehicle control systemsCAVCS according to the present invention.

This CAVCS includes the tire force determining system TFDS on each ofthe four tires, and, as the main unit, at least one of anti-lock brakesystem (ABS), traction control system (TCS), vehicle stability controlsystem (VSCS), attitude control system (ACS), suspension control system(SCS), steer-by-wire system (SBWS) and the like is included.

The tire force determining system TFDS comprises:

-   one or more tire strain sensors S disposed on a tire;-   a data processor DPU computing tire force F based on the data on the    tire strain obtained from the sensors S; and-   a tire sensor controller TSC receiving the outputs of the sensors S    representing the tire strain, and outputting or forwarding the data    on the tire strain towards the main unit of CAVCS in order.

The tire sensor controller TSC comprises an amplifier AMP and atransmitter TR.

As to the amplifier AMP, the analog output of a sensor S is usually verysmall. Therefore, a linear amplifier is included. If the sensor outputis nonlinear, a nonlinear or equalizer amplifier can be used instead. Ifnecessary, an analog to digital converter ADC is included in theamplifier AMP. In the following embodiments, an ADC is included.

The data processor DPU comprises a receiver RE, a processor CPU, amemory MEM, I/O device.

In the following embodiments, the tire force determining system TFDS oneach tire is depicted as if each TFDS includes a processor CPU, a memoryMEM and the like. But, this is only for the sake of simplicity. Forexample, it is possible to assign one processor CPU and one memory MEMto TFDS for all the tires. Further, it is also possible to use theprocessor CPU of the main unit of the CAVCS, e.g. ABS, VSCS and thelike.

<Method of Determining the Force>

First, a method of determining the forces is explained. Such method isstored in the memory as a computer program and performed by theprocessor CPU.

In the above-mentioned zone Y, a tire strain ∈ caused by a force F isapproximately expressed as a linear function of the force F such that∈=f(F).

For example, a tire strain ∈x caused by a back-and-forth-direction forceFx is approximately expressed as a linear function of the force Fx suchthat∈x=f(Fx),a tire strain ∈y caused by a lateral force Fy is approximately expressedas a linear function of the force Fy such that∈x=f(Fy), anda tire strain ∈z caused by the vertical force Fz is approximatelyexpressed as a linear function of the force Fz such that∈x=f(Fz).Accordingly, the tire strain ∈ caused by a resultant force F of thethree translational forces Fx, Fy and Fz can be given as the sum of thestrains ∈x, ∈y and ∈z and expressed as follow:∈=∈x+∈y+∈z=f(Fx)+f(Fy)+f(Fz)Thus, the values ti (i=1 to 3) of the tire strain ∈ observed atdifferent measuring points MPi are:t1=A1*Fx+B1*Fy+C1*Fzt2=A2*Fx+B2*Fy+C2*Fzt3=A3*Fx+B3*Fy+C3*Fzwherein

Ai, Bi and Ci (i=1 to 3) are coefficients at the measuring points MPi(i=1 to 3).

Thus, by solving these simultaneous equations, the unknown variable Fx,Fy and Fz can be obtained.

The simultaneous equations are expressed by the following determinant:

$| \begin{matrix}{t\; 1} \\{t\; 2} \\{t\; 3}\end{matrix} | = | \begin{matrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3}\end{matrix} \middle| \; \middle| \begin{matrix}{Fx} \\{Fy} \\{Fz}\end{matrix} |$Accordingly, by computing the following determinant

$| \begin{matrix}{Fx} \\{Fy} \\{Fz}\end{matrix} | = | \begin{matrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3}\end{matrix} \middle| {}_{- 1} \middle| \begin{matrix}{t\; 1} \\{t\; 2} \\{t\; 3}\end{matrix} |$Fx, Fy and Fz can be easily obtained.<General Expression>

In general, as explained above, as far as the correlation issubstantially linear, a tire strain ti at measuring point MPi (i=1 tom), forces Fj (j=1 to n) to seek and coefficients Kij at a measuringpoint MPi for the forces Fj are formularized as follows:t1=K11*F1+K12*F2+K13*F3- - - K1n*Fnt2=K21*F1+K22*F2+K23*F3- - - K2n*Fnt3=K31*F1+K32*F2+K33*F3- - - K3n*Fntn=Kn1*F1+Kn2*Fn+Kn3*F3- - - Knn*Fntm=Km1*F1+Km2*Fn+Km3*F3- - - Kmn*FnThus, the forces F1, F2- - - Fn can be determined from the followingdeterminant.

$| \begin{matrix}{F\; 1} \\{F\; 2} \\{F\; 3} \\\; \\{Fn}\end{matrix} | = | \begin{matrix}{K\; 11} & {K\; 12} & {K\; 13} & {-- -} & {K\; 1\; n} \\{K\; 21} & {K\; 22} & {K\; 23} & {-- -} & {K\; 2\; n} \\{K\; 31} & {K\; 32} & {K\; 33} & {-- -} & {K\; 3\; n} \\{K\;{n1}} & {K\;{n2}} & {K\;{n3}} & {-- -} & {Knn} \\{{Km}\; 1} & {{Km}\; 2} & {{Km}\; 3} & {-- -} & {Kmn}\end{matrix} \middle| {}_{- 1} \middle| \begin{matrix}{t\; 1} \\{t\; 2} \\{t\; 3} \\{tn} \\{tm}\end{matrix} |$<Measuring Point>

The above-mentioned measuring points MP are, as shown in FIG. 1, fixedpoints on a coordinate system which is bounded to the tire equatorialplane not rotated if the tire rotates. Therefore, when the tire strainis measured with a sensor S fixed to the tire 1, it is necessary tolocate the sensor S because the sensor moves around the rotational axisof the tire as the tire rotates.

In general, deformation of a pneumatic tire is larger in a lower half ofthe tire than an upper half. Therefore, it is preferable that themeasuring points are set in a lower half rather than an upper half.

In case that the forces to seek are Fx, Fy and Fz, the measuring pointsMP are preferably three fixed points as shown in FIG. 14 by darkrectangles: a point MP[0] at the center (0 deg.) of the groundcontacting patch, and two symmetric positions MP[90] and MP[270] (90 and270 deg.) at an angle alpha of 90 degrees from the center (0 deg.).Aside from 90 degrees, the angle alpha may have another values. However,if the measuring points MP at which the sensors S measure the tirestrain at a time are too near, the resolution between the forces becomesdifficult. Therefore, as shown in FIG. 14, the angle alpha between theadjacent measuring points around the tire rotational axis is at least 30degrees, preferably at least about 60 degrees (a lower half) but at most120 degrees (an upper half).

On the other hands, according to the number (n) of the forces to seek,more than three measuring points, for example eight measuring points asshown in FIG. 14 by rectangles (dark rectangles and white rectangles),are possible. In this case, the above-mentioned limitation to the anglealpha is not applied.

The above-mentioned inverse matrix or the coefficients Ai, Bi and Ci(generally Kij) varies depending on the measuring points MP. Therefore,in order to compute the forces Fx, Fy and Fz (generally Fj), it isnecessary to obtain and store the coefficients or data thereonbeforehand.

<Eight-sensor Arrangement>

As the tire rotates, it is necessary to locate the sensors' positions ofthe rotating tire, and to read the sensors' outputs at the substantiallysame moment when the sensors come to the measuring points.

FIG. 15 shows an example of the sensor arrangement on the tire 1,wherein eight sensors S are disposed equiangularly around the tirerotational axis. This sensor arrangement can be combined with theabove-mentioned three measuring points MP[0], MP[90] and MP[270] shownin FIG. 14.

<Examples of TFDS>

Firstly, explained is a system designed for such case that the measuringpoints MP are fixed points, and the number of the sensors S is more thanthe number (m) of the measuring points MP. The number (n) of the forcesFj is of course not more than the number (m).

FIGS. 16 and 17 each shows a system for determining the translationalforces Fx, Fy and Fz for a combination of the eight-sensor arrangementand three measuring point arrangement.

First, the above-mentioned coefficients Ai, Bi and Ci (i=1 to 3) atevery measuring point MP[0], MP[90] and MP[270] are determined throughmeasurements, and the date on the coefficients Ai, Bi and Ci are storedin the memory MEM for example in a form usable as the inverse matrix.

The outputs of the sensors S representing tire strain are forwardedthrough the amplifier AMP, and are encoded/modulated to send out by thetransmitter TR.

The transmitted signal is received and decoded/demodulated by thereceiver RE, and outputted towards the processor CPU.

AS to the transmitted data, it is possible to send out (1) data of allthe sensors S, or alternatively (2) data of selected sensors Spositioned at the measuring points MP. In the first case (1), as shownin FIG. 16, the data relating to the measuring points are selected fromall the data with a selector SW on the receiving side.

In the latter case (2), as shown in FIG. 17, the data relating to themeasuring points are selected from all the data with a selector SW onthe transmitting side.

In case that the selector SW selects analog data, the selector SW may bean assembly of semiconductor switches. In case of digital signal, theselector SW may be a set of logical circuits or programmed IC.

In any case, according to data relating to the position of at least onesensor S, the selector SW selects and outputs only the data relating tothe relevant measuring points MP as the above-mentioned values ti (i=1to 3) representing the tire strain ∈.

The data relating to the position of at least one sensor S is providedby a sensor locator SL as the sensor locating data. The sensor locatorSL outputs sensor locating data from which the positions of the sensorsS can be determined.

In the FIG. 16 example wherein the selector SW is provided on thereceiving side (vehicle body side), for example, a rotary encoderconnected to the axle of the vehicle can be used.

In case of FIG. 17, the above-mentioned rotary encoder can be used.Further, as shown in FIG. 18 and FIG. 19, it is also possible to employsuch a novel method that an induction coil COL connected to anoscillator OSL generating a specific frequency fs (for example severalhundred herz) is disposed at a point not the measuring points forexample at a point MP[180] in FIG. 14. Therefore, when one of thesensors S comes to near the induction coil COL, the sensor S outputs ananalog signal including an alternate current of frequency fs althoughthe other sensors do not. Thus, the positions of all the sensors S canbe easily located. A band pass filter BP tuned to the frequency fs andconnected to each of the sensors' analog signal lines is incorporated inthe amplifiers AMP and the band pass filters' outputs are provided forthe selector sw as the above-mentioned data relating to the position ofat least one sensor S.

Using the date on the tire strain ti, the coefficients Ai, Bi and Ci andso on, the processor CPU computes the forces and outputs data thereontowards the main unit of the CAVCS.

As to the number of the sensors S, in case that the forces to seek arethree forces in three translational motion directions, at least threesensors are required to determine the forces. In view of accuracy,however, at least four, preferably at least six, more preferably atleast eight sensors S are used.

If the forces to seek are the three forces Fx, Fy and Fz plus threerotational forces Fyz, Fzx and Fxy around the axes x, y and z of thetranslational motion directions, at least six sensors S are required todetermine these six forces. Preferably eight or more sensors are usedfor accurately determining the forces.

In the above-mentioned examples, the number of the sensors S is morethan the number of the forces to seek, and the output data (t) from thesensors are selected with the selector SW, according to the sensorlocation data (SL), to be decreased to the number of the forces.

In the examples shown in FIG. 20 and FIG. 21, the selector SW, afunction to select the sensors at the measuring points, is omitted.

In FIG. 20, the number of the sensors S equals to the number of themeasuring points. In this example, the eight sensor arrangement shown inFIG. 15 and the eight measuring point arrangement shown in FIG. 14 arecombined.

In this case too, similarly to the former examples, the coefficients(A0-A8, B0-B8 and C0-c8) at each measuring point MP[0], MP[45], MP[90],MP[135], MP[180], MP[225], MP[270] or MP[315] are stored in the memoryMEM, and the processor CPU computes the forces Fx, Fy and Fz accordingto data t1-t8 on the tire strain from the sensors S1-s8, the sensorlocation data output from the sensor locator SL, and the above-mentionedcoefficients.

<Further Example of TFDS>

In the above mentioned examples, the measuring points are fixed points.These measuring points are regarded as one set as the tire strainsthereat are measured at a time.

Thus, the intervals at which the computed forces can be obtained isbasically determined by the number of the sensors S. If the sensors'number is decreased, as the intervals becomes long, and it becomesdifficult to obtain the necessary data in time at high-speed rotation.Contrary, at a slow-speed rotation, a time period for which thecomputation of the forces is impossible becomes long. When eight sensorsare disposed equiangularly, the computed forces are obtained every 45degrees. Although, the intervals can be shortened by increasing thenumber of the sensors S, the increasing of the sensors S is notpreferable. Thus, the number is usually at most 16, and at least 4preferably 8.

The following is a method for shortening the intervals withoutincreasing the number of the sensors.

The intervals can be shortened by increasing the number of measuringpoints MP and use different sets of measuring points as the tirerotates. In other words, the measuring points at which the sensorsmeasure the tire strain are changed as the tire rotates.

FIG. 21 shows a system designed for such case that the number (m) of themeasuring points MP is more than the number of the sensors S.

In this embodiment, the forces to seek are Fx, Fy and Fz, and eightsensors S1-S8 are disposed equiangularly.

The number of the measuring points is very large, for example, 360. Inother words, the measuring points are provided around the rotationalaxis every 1 degree.

The output data t1-t8 of all the sensors S1-s8 are transmitted to theprocessor CPU through the amplifiers AMP, the transmitter TR, receiverRE, etc.

The sensor locator SL outputs data from which the positions of thesensors S can be determined. For example, the selector sw is a rotaryencoder connected to the axle of the vehicle. According to the outputdata of the sensor locator SL, the processor CPU reads the memory forthe stored data on the coefficients at the located measuring points and,using those data, the CPU computes the forces Fx, Fy and Fz.

Thus, in this embodiment, the forces can be obtained every 1 degree oftire rotation.

In this embodiment however, the record data size of the coefficients Kijbecomes large because eight coefficients K(1-360)(1-8) are necessary pereach of the 360 measuring points MP[0]-MP[359]. Thus, it is preferredthat the record data size is decreased as follows.

For example, the tire strain in an upper half of the tire is not sosignificant for the forces Fx, Fy and Fz. Therefore, the measuringpoints can be provided in only a lower half. Thus, the number and sizeare decreased to one half. In this case, since four sensors S stillexist in the lower half, accurate analysis is possible every 1 degreerotation.

<Linear Interpolation>

Further, without changing the intervals (in this example ever 1 degree),the record data size can be further decreased by the following method.This method can be adopted regardless of whether the area of themeasuring points is a lower half, namely, whether partial or full. Themeasuring points (for example every 1 degree) are not changed, but thestored coefficients K are decreased to for example every 2 or moredegrees, and coefficients at missing measuring points are generated by,for example, a linear interpolation as follows.

Given that K[θl] is the coefficient at a missing measuring positionMP[θl], K[θk] is the coefficient at an adjacent measuring positionMP[θk] stored, K[θm] is the coefficient at an adjacent measuringposition MP[θm] stored, and θ is an angle, for example, based on thecenter of the ground contacting patch being zero (θk<θl<θm), K[θl] isdetermined as follows:K[θl]=K[θk]*(θm−θl)/(θm−θk)+K[θm]*(θl−θk)/(θm−θk)Thereby, the data can be reduced to 50% to about 20%.<Method of Obtaining Coefficients>

As to the coefficients, the coefficients at each measuring point aredetermined through tire loading tests as follows. The tire is mounted onthe wheel rim and inflated to a normal or standard pressure. Applying aspecific tire force such as Fx, Fy, Fz in turn and changing themagnitude thereof, the tire strain is actually measured with the sensorsS, and the coefficients are determined through analyzing of the sensors'outputs. More specifically, by making a regression analysis, taking theforces (Fx, Fy, Fz) as independent variables and the sensor outputs (t1to t3) as dependent variables, the coefficients (A1-A3, B1-B3, C1-C3)can be obtained as the regression coefficients. The coefficients arestored in the memory MEM.

<DC Power>

As to the electric power for the sensors S, amplifiers AMP, transmitterTR and so forth provided on the tire side, a combination of a wirelesspower transmit system, for example utilizing electromagnetic induction,electromagnetic wave or the like, and an electric accumulator ispreferred for ease of maintenance or maintenance free. However, if therequired total power is large, the electric power can be suppliedthrough a power transmission unit utilizing physical contact betweenelectric conductors such as slip ring unit. In this case, in order tosend the sensor data from the vehicle wheels (tires) to the vehiclebody, the same physical contact type power transmission unit can beused. More specifically, the transmitter TR generates a carrier wavemodulated by the sensor data and superposes the modulated carrier waveon the DC power. The modulated carrier wave is split off from the DCpower and demodulated into the sensor data by the receiver RE.

<Mounting Position of Tire Sensor Controller>

The tire sensor controller TSC is fixed to the tire 1 as shown in FIGS.10 and 12 or wheel rim 17 as shown in FIG. 11 using adhesive agent,bracket and the like. In view of tire performance and characteristics,it is preferable that the controller TSC is fixed in the wheel rim well,or to the inner surface of the vulcanized tire at a position near thebottom of the bead portion 4. But it may be also possible to fix toanother part as far as the part rotates together with the tire andwheel. In any case, in particular FIGS. 10 and 11 cases, the lead wires16 between the strain sensors S and tire sensor controller TSC arepreferably embedded in tire rubber before tire vulcanization.

1. A system for determining the magnitude of a force acting on a tire,comprising the tire, a tire strain sensor mounted on the tire, fordetecting a tire strain at the mounted position, and generating datarepresenting the tire strain, wherein said mounted position is a fixedposition on the tire, a sensor locator for locating the sensor on thetire while the tire rotates together with the sensor, a memory in whichdata on relationships between the tire strain and force acting on thetire at each of measuring points are stored, and a processor computingthe magnitude of the force, using the data representing the tire strainobtained from the tire strain sensor and the data on the relationshipsacquired from the memory based on the data on the sensor locationobtained from the sensor locator, whereby the magnitude of the forceacting on the tire is determined by utilizing the tire strain caused bythe force.
 2. The system according to claim 1, wherein said mountedposition is a position where the force and resultant tire strain have asubstantially linear correlation.
 3. The system according to claim 2,wherein said mounted position is in the sidewall portion of the tire. 4.The system according to claim 1, wherein said tire sensor is disposed ineach of circumferentially different positions, and the number of thesensors is at least three.
 5. The system according to claim 4, whereinthe number of the strain sensors is at least six.
 6. The systemaccording to claim 5, wherein said force is a force in each of threetranslational motion directions, and the processor computes themagnitude of each force, using the data representing tire strainsobtained from the tire strain sensors and the data on the relationshipsacquired from the memory based on the data on the sensor locationsobtained from the sensor locator.
 7. The system according to claim 1,wherein the strain sensor has a directivity and is mounted so that thedirectivity inclines with respect to the tire radial direction.
 8. Thesystem according to claim 1, wherein the strain sensor comprises atleast one magnet and at least one magnetometric sensor embedded in aresilient material.
 9. The system according to claim 1 or 8, wherein thestrain sensor has the most sensitive direction, and the most sensitivedirection is inclined at an angle of from 10 to 80 degrees with respectto the tire radial direction.
 10. A method of determining the magnitudeof a force acting on a tire, comprising obtaining data on a tire strainfrom a tire strain sensor mounted on the tire, locating the tire strainsensor, to obtain data on the position of the tire strain sensor,obtaining data on relationships between the tire strain and force at thelocated position of the tire strain sensor, from previously stored dataon relationships between the tire strain and force at a plurality ofpositions, and computing the force using the obtained data on the tirestrain and the obtained data on the relationships between the tirestrain and the force.
 11. The method according to claim 10, wherein atleast three tire strain sensors are disposed circumferentially of thetire, and in the process of obtaining the data on the tire strain, thedata are obtained from each of three or more of said at least three tirestrain sensors, in the process of locating the tire strain sensor, dataon the position of each said three or more of the tire strain sensorsare obtained, in the process of obtaining the data on the relationshipsbetween the tire strain and the force, the relationships for each ofsaid three or more of the tire strain sensors are obtained, in theprocess of computing the force, at least three forces are computed,using the obtained data on the tire strains and the obtained data on therelationships.